As the purity of seized cocaine has declined in recent years, ‘it would come as little surprise if users turned their backs on a low-quality product’, said experts.

Previously, cocaine use had risen steadily every year since 2005, but the number of 18 to 24-year-olds seeking treatment fell substantially this year, down 14 per cent to 7,304 from 8,522 in 2008-09.

‘Despite this apparent step away from the most harmful street drugs, there is some evidence of a corresponding move towards new synthetic compounds such as mephedrone,’ the National Treatment Agency for Substance Misuse said.

‘With legal highs, there have been lots of reports of different side effects like people committing suicide or becoming paranoid because of mephedrone binges. It’s new so we are still trying to establish the best way to deal with it.’

Mephedrone – also known as meow meow, bubbles and M-cat – was banned and made a class B drug in April after it was linked to the deaths of two teenagers.

It later transpired they had not taken the drug.

The numbers seeking treatment for crack and heroin addiction were also down 40 per cent on figures for 2005.

‘The figures confirm the NTA’s claim that the ‘Trainspotting generation’ that got hooked in the 1980s is growing older, fewer young people are risking getting addicted to heroin, and treatment is beginning to show an impact on drug use,’ said NTA chief executive Paul Hayes.

The only drug which saw a rise in use was cannabis, which accounted for 29 per cent of all new treatment cases, up from 18 per cent five years ago.